Mirrors used in the projection optical systems of extreme ultraviolet lithography (EUVL) scanners include a reflective coating on a substrate. Since the illumination sources used in EUVL are high power, mirror substrates must meet stringent thermal expansion requirements in order to maintain their intended surface shape (known as “figure”) when subjected to the temperature changes associated with the high thermal loads of the illumination source during normal operation of the scanner. A temperature-stable figure is necessary to avoid thermally-induced distortions in the wavefront characteristics of EUV projection optics. For this reason, the preferred material for manufacturing state of the mirror substrates for EUV projection optics is Ultra Low Expansion glass (ULE® Glass), manufactured by Corning Incorporated. Glass sold by Corning Inc. under the product code 7973 is specifically tuned for EUVL applications and is characterized by high degrees of precision and accuracy in controlling properties to achieve glasses with properties that can be customized to diverse applications. A defining feature of ULE® glass is the existence of a temperature close to room temperature at which its coefficient of thermal expansion (CTE) is exactly equal to zero. This temperature is known as a crossover temperature, a zero crossover temperature, or a temperature of zero crossover, and is denoted Tzc. Another important feature of ULE® glass for EUVL is that the slope of the temperature-dependent CTE curve (CTE slope or expansivity slope) is extremely small within a temperature range close to room temperature that includes Tzc. The CTE slope of ULE® glass is in the vicinity of 1×10−9/K2 (or, equivalently, 1 ppb/K2). EUVL mirror substrates having Tzc near the temperatures expected when the mirror substrate is exposed to an EUV illumination source experience minimal thermal expansion during operation of the EUVL scanner and a small CTE slope ensures that the minimal thermal expansion is preserved if fluctuations in EUVL processing conditions cause variations in the thermal environment of the mirror substrate.
There are two sources of distortion of figure as mirrors are heated up: (1) if the mirror is heated uniformly (i.e., the temperature is uniform throughout its volume), its shape will change if the CTE is not uniform, because each volume element within the mirror substrate will seek to expand according to its local CTE value; and (2) if the CTE of the substrate is perfectly uniform, the figure of the substrate will become distorted if the substrate is heated non-uniformly. While the first source of distortion can be minimized by providing a highly uniform substrate material, minimization of the second source of distortion requires the value of CTE to be as close to zero as possible throughout the temperature range in use. Near-zero CTE can be achieved by choosing a substrate material with Tzc within the expected operational temperature range and minimizing the slope of the expansivity (CTE vs. T) curve. Actual EUVL mirror substrates suffer distortions due to both sources: the substrate material is never perfectly uniform and the mirrors are heated non-uniformly in practical operation. In order to maximize performance during use of the mirrors, it is therefore important to simultaneously achieve a high degree of CTE uniformity, a highly accurate Tzc targeted to the anticipated operational temperature range, and a CTE slope that is as low as possible.